In a time-of-flight mass spectrometer (which is hereinafter abbreviated as “TOFMS”), a specific amount of kinetic energy is imparted to ions originating from sample components to make the ions fly over a specific length of space. The period of time required for the flight is measured for each ion, and the mass-to-charge ratio of each ion is determined from the time of flight of that ion. Therefore, one of the major causes of a decrease in the mass-resolving power in the TOFMS is a variation in the initial energy of the ions. To address this problem, a reflectron-type TOFMS includes a reflectron having the function of correcting the difference in the kinetic energy of the ions. Though no detailed description will hereinafter be made, a commonly known, dual-stage reflectron is capable of up to the second-order energy focusing (i.e. the distribution of the time of flight can be corrected up to the second-order derivative of the energy). Therefore, even if there is a certain amount of variation in the kinetic energy of the ions, the reflectron can correct that variation and focus the ions within a certain range of time of flight, thus preventing the deterioration in the mass-resolving power.
Another cause of the deterioration in the mass-resolving power is the turn-around time which occurs in a system that captures ions in an ion trap or similar device and imparts an amount of acceleration energy to the captured ions to bring them into flight. When ions are accelerated in the time-of-flight analysis direction, an ion having a velocity component opposite to the time-of-flight analysis direction due to its initial energy consumes a certain amount of time to return to the starting point after departing from the starting point. This is the turn-around time. It corresponds to the time-of-flight difference between an ion having a forward velocity component with respect to the time-of-flight analysis direction and an ion having an opposite velocity component. Accordingly, in a broad sense, the turn-around time can also be regarded as a resultant of the variation in the initial energy of the ions. However, the error due to the turn-around time cannot be corrected by a reflectron. Therefore, how to reduce the influence of the turn-around time is an essential problem for the mass-resolving power of the TOFMS.
One technique for solving this problem is an orthogonal acceleration TOFMS in which ions are accelerated in a direction orthogonal to the incident direction of an ion beam and sent into a time-of-flight analysis space (for example, see Patent Document 1 or Non-Patent Document 1). FIG. 11 is a schematic configuration diagram of an orthogonal ion acceleration unit and an ion-injecting optical system located before that unit in an orthogonal acceleration TOFMS.
An orthogonal acceleration unit 4 includes a plate electrode 41 and a mesh electrode 42 having a large number of openings for allowing ions to pass through, and an ion-injecting optical system 300 including a beam-restricting mechanism consisting of two slit plates (or aperture plates) 301 and 302 separated by a predetermined gap L. In this figure, the initial direction of the ion beam entering the accelerating region between the electrodes 41 and 42 is the X direction, while the accelerating direction (i.e. the time-of-flight analysis direction) is the Z-direction orthogonal to the X-direction. When ions are injected from the beam-restricting mechanism into the orthogonal acceleration unit 4, the electrodes 41 and 42 are at the same potential (e.g. ground potential) and no electric field is present in the accelerating region. When an adequate amount of ions have been injected, a high-voltage pulse having the same polarity as the ions is applied to the plate electrode 41, whereby an accelerating electric field is created in the accelerating region, imparting a large amount of kinetic energy to the ions. As a result, the ions begin to fly, passing through the openings of the mesh electrode 42.
A time-of-flight distribution in the orthogonal acceleration unit 4 is hereinafter discussed.
An initial energy Ez of the ions in the time-of-flight analysis direction is given by Ez=E sin2α, where E is the amount of energy of an ion beam entering the orthogonally accelerating region and α is the angle to the X axis of the beam. The higher the initial energy Ez is, the larger the time-of-flight distribution due to the aforementioned turn-around time is. To decrease the initial energy Ez, it is necessary to reduce the amount of energy E and the angle α. The beam-restricting mechanism is aimed at limiting this angle α to a small value. In the example of FIG. 11, the angular spread of the beam is given by α=tan−1(h/L), where L is the gap between the two slit plates 301 and 302, and h is the aperture width of the slit plate 302. Therefore, it is possible to decrease the angle α of the ion beam and reduce the dispersion in the initial energy of the ions within an allowable range by appropriately setting the gap L and the aperture width h.
In a system described in Patent Document 1 and other documents, an electrostatic lens which is an aperture lens is provided between an ion trap and a beam-restricting mechanism in order to efficiently introduce ions released from the ion trap. Such a combination of the electrostatic lens in the form of an aperture lens and a beam-restricting mechanism consisting of a pair of slit plates have been widely used in practical systems.
However, the previously described conventional configuration has the following problem:
In the aforementioned beam-restricting mechanism, a considerable portion of the ion-beam flux collides with and is blocked by the slit plate. Therefore, the amount of ions actually used for the time-of-flight analysis is considerably smaller than the original amount, so that the measurement sensitivity inevitably deteriorates. To improve the measurement sensitivity, it is necessary to increase the aperture width h of the slit, which, however, increases the angle α of the beam and lowers the mass-resolving power. Thus, there is a trade-off between the mass-resolving power and the measurement sensitivity, and it is unavoidable to sacrifice the measurement sensitivity if a high level of mass-resolving power needs to be achieved.
In the aforementioned conventional configuration, the mass-resolving power is determined by the gap between the two slit plates and the aperture width of the slit. Therefore, for example, when a high-sensitivity measurement must be performed while allowing a slight deterioration of the mass-resolving power, it is necessary to perform a mechanical task, such as replacing the slit plates of the beam-restricting mechanism with the ones having a different aperture width or adjusting the gap of the slit plates. Such a task is cumbersome and time-consuming. Furthermore, a mechanism which allows such a mechanical adjustment or replacement has a problem in terms of reliability.